Plants are fierce competitors

June 2, 2013 lawanda Newspaper Columns

Competition in the plant world is fierce.  The techniques plants use against each other to ensure their own survival make the conniving in corporate boardrooms and reality TV shows look like child’s play.

Some plants develop extremely dense or deep roots systems so they can steal water and nutrients from seedlings of any other species that might be trying to establish nearby.  Others produce hundreds of seeds per plant to saturate the surrounding area with their offspring.  Still others leaf out early and grow extremely fast in an effort to starve competitors of sunlight.

The most insidious method plants use to prevail over competitors for water, nutrients, space and sunlight, however, is chemical warfare.  They emit chemicals into the soil from their roots that inhibit the germination or growth of other plants.  The phenomenon is called allelopathy (pronounced a-LEE-la-path-ee).

The plant most famous for this is the black walnut tree, which emits a chemical called juglone.  Actually the black walnut does more than just secrete juglone from its roots.  It  is exuded from the bark, stems, leaves, roots and nuts, and is even in rainwater that drips from the tree.

Juglone does not move far once in the soil, but even small amounts may injure or kill sensitive plants.  Plants whose roots get within a half to a quarter inch of walnut roots may encounter juglone.  Roots of walnut trees can extend far beyond the tree’s drip line so they really keep their rivals at bay.

Other trees that produce juglone are red oak, tree of heaven, black cherry, sugar maple, hackberry, American sycamore and cottonwood.

What if you love your juglone-producer and don’t want to cut it down?  As a side note, it might not matter anyway, because juglone persists in soil for 5-20 years, depending on conditions, after a tree is felled.

To start, there is a long list of plants that can stand against juglone and do just fine.  You can find a list of the most juglone-sensitive and the most juglone-tolerant plants here:

If you must locate your garden near a juglone emitter, a raised bed constructed to minimize root penetration will help.  Still, you must take care to keep the beds free of leaf litter and nuts.  Studies show decreased juglone toxicity with well-drained soil so anything you can do to increase drainage such as adding organic matter or replacing the current soil with a lighter type will help.

Leaves, bark, and wood chips of juglone producing plants should not be used for mulch around landscape or garden plants.  Even composting will not entirely remove the juglone.

You might have noted the use of the word “toxicity” in relation to juglone.  Don’t worry, it is toxic to plants, not to humans.  However, horses may have quite severe symptoms when black walnut wood shavings are used as bedding and dogs can have gastric reactions if they eat the hulls from the nuts.


MiscellaneousTrees and Shrubs

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